1. Field of the Invention
The present invention relates generally to telephone transmission and, more particularly, to the efficient transmission of high-speed digital signals between a telephone central office and the customer premises.
2. Description of the Related Art
Perhaps the most flexible and least expensive approach for transmitting data over telephone lines is to use the existing voiceband telephone channels normally used to carry speech. The channel between the transmission endpoints may be either a switched network connection which may be established by the user at one endpoint by simply dialing the telephone number of the other endpoint, or it may be a permanent, private line connection which is set up for the user by the telephone company. In either case, once the connection has been established, data from the user's data communication processing equipment is input to a voiceband modem which generates an output analog line signal having a frequency spectrum which matches the passband of the voiceband telephone channel. At the receiving end, a matching modem recovers the data from the received line signal and passes it to the user's equipment at that end.
For a given level of noise and distortion, the rate at which data can be communicated over a channel is limited by its bandwidth and noise, including distortion, within the channel. The bandwidth of the typical voiceband telephone channel is about 4 kHz. For typical levels of noise, this limits the transmission rate over such channels to a theoretical maximum to about 20 to 30 kilobits per second (kb/s). For many applications, such as database input or retrieval or other applications typically involving a human being at one end of the transaction, these data rates are wholly satisfactory.
For many other applications, however, such as computer to computer file transfers, videotext, transmission of digitized speech or video and the like, voiceband telephone data transmission is unacceptably slow. Advantageously, most of the transmission facilities interconnecting telephone switching offices communicate their information in the form of multiplexed, high speed digital bit streams. These facilities can be configured to provide not only the standard 4 kHz voiceband channels, but also wideband channels capable of carrying customer data at, for example, the so-called DS-1 rate of 1.544 megabits per second (Mb/s) and higher.
The challenge, however, is to get the customer's high-speed data to the central office, and high speed data from the central office to the customer. In the future, this may be accomplished by linking customer premises with central offices through optical fiber. However, it could be well into the twenty first century before such a system is put in place. For the immediate future, the existing telephone local distribution system-comprising copper wire pairs will continue to be the physical medium for delivering high-speed data to customer premises.
Telephone engineers have been successful in providing transmission schemes that allow for high-speed data transmission from customer locations to the central office. In the mid-1970's, for example, AT&T introduced a digital data communications network, the Digital Data System (DDS), in which data at rates up to 56 kb/s was transmitted from the customer locations to the central office using a four-wire local circuit, that is, two two-wire pairs. The essence of the transmission scheme was to use bipolar baseband transmission in combination with, inter alia, fixed equalization to compensate for linear distortion and thereby provide a channel with flat loss up to frequencies sufficient to transmit at the required bit rate. This scheme allowed for transmission over distances of almost eight miles at the 56 kb/s rate and even greater distance at lower rates without the use of repeaters, thereby providing high-speed customer-premises-to-central-office transmission over the "local loop" for a significant base of customers. See, for example, E. C. Bender et al., "Digital Data System: Local Distribution System", The Bell System Technical Journal, Vol. 54, No. 5, May-June 1975, which is incorporated herein in its entirety.
Subsequently, a 1.544 Mb/s speed was added to DDS, and data transmission at that rate was thereafter provided in other digital data transmission offerings. This transmission rate was achieved by using the technology developed for the so-called T1 carrier system which had to that point been principally used to interconnect telephone central offices. Here again, the transmission scheme involved a four-wire circuit and a bipolar transmission format. Indeed, the design of DDS was based on the previously existing T1 technology. At the 1.544 Mb/s rate, however, compensation for channel distortion and noise required equalization and regeneration of the line signal typically at every 3000 ft (6 kft).
The above approaches are certainly technically sound and are used quite extensively. However, not only is transmission based on T1 technology relatively expensive to provide and maintain see for example, Method and Apparatus for Wideband Transmission of Digital Signals Between, For Example, a Telephone Central Office and Customer locations, U.S. Pat. No. 4,924,492 issued to Gitlin et al., which is hereby incorporated by reference, more demanding applications for the telephone network have arisen. Although video on demand services, for example, can be accommodated within the framework of Asymmetric Digital Subscriber Line services (ADSL), at a data rate of only 1.544 Mb/s see for example, "PSTN Architecture For Video On Demand Services", U.S. Pat. No. 5,247,347 issued to Litteral et al., which is hereby incorporated by reference, even higher data rates will be required for some applications. Very high data rate subscriber line (VDSL) systems will address the requirements of these applications such as the delivery of high definition television. Further, the trend has been that data rates considered high today are considered low several years later.
As noted above, at some point a high bandwidth medium, such as optical fiber, may very well provide a communications path from every telephone operating company to every customer location, thus allowing high speed data communications through the telephone network. In the interim, however, VDSL systems may employ a mix of technologies to establish high speed communications to every customer location. A two wire pair can support data rates up to 51.84 Mb/s, 25.92 Mb/s, or 12.96 Mb/s for respective distances of 1 kft, 3 kft, and 4.5 kft. Because the ubiquitous two wire pairs currently provide connection from most customer locations to the PSTN, it would be advantageous to capitalize on this enormous installed base. That is, rather than incurring the expense and inconvenience associated with replacing all two wire pairs with optical fiber, telephone companies could employ optical fiber to distribute data to a point within the range of twisted pairs for a desired data rate. From this point, a distribution cable containing several twisted pairs could connect to individual premises within the neighborhood. In some cases, the entire neighborhood may be within the desired two-wire range of a central office, for example, within 3 kft where 25.9 Mb/s services are desired. In those cases, the distribution cable could run directly from the central office to customer locations. For those situations where the customer locations are outside the desired two-wire range, an optical fiber could connect the central office to an optical network unit which would provide an interface between the optical fiber and one or more two-wire distribution cables.
Generally, it would be desirable to support both symmetric and asymmetric services within such a neighborhood and, therefore, within a single distribution cable. Asymmetric services would accommodate such applications as video on demand, in which an upstream channel, that is the channel from the customer locations to the telephone network, requires substantially less bandwidth than a downstream channel, or the channel from the network to the customer location. Symmetric services would be directed toward applications, such as working at home, which requires inter-computer communications, in which the upstream and downstream channels require substantially the same bandwidth. Additionally, it would be desirable to provide flexibility to telephone companies and their customers by supporting various asymmetric ratios within a given cable.
One of the difficulties that arises when attempting to provide such a variety of services within a single cable is that the information carrying capacity of any channel is limited by the channel's bandwidth and noise. In the context of two wire local loops, near-end cross talk, that is, interference from a transmitter at one end of the cable with a receiver at the same end of the cable, has the greatest potential for degradation of the channel's information capacity. One approach to providing duplex operations for a given distribution cable is time division duplexing (TDD). In a time division duplexing system, near-end cross talk is virtually eliminated by insuring that no transmitter transmits at the same time a receiver at its end of the cable is receiving. Analogously, in a frequency division duplexing (FDD) system, near-end cross talk is substantially reduced by insuring that a transmitter employs a different frequency band to transmit than the frequency band receivers at the same end of the cable use for receiving.
There are advantages and disadvantages to both FDD and TDD systems. For example, a TDD system must synchronize frames across an entire cable in order to reduce near-end cross-talk, but an FDD system need not do so. Additionally, since the loss of synchronization in a two-wire pair could seriously degrade service in adjacent pairs within a cable, a TDD system is generally more vulnerable to faults in a communications system's digital electronics. Another way in which FDD systems are generally more attractive than TDD systems involves the fact that, although crosstalk can be substantially reduced, it is generally not entirely eliminated. This residual crosstalk can be demodulated by a nonlinearity on a telephone line to produce noise. Noise from a FDD system would sound like stationary white noise to a human listener, but noise from a TDD system would sound like white noise modulated by on/off keying with a cycle rate near the peak response of the human ear. Additionally, any attempt at mixing symmetric and asymmetric services in a TDD system would seriously degrade the available data rate of the symmetric services.
For the above reasons an FDD system may appear to be the system of choice, but FDD system implementations are not without difficulty. Since symmetric and asymmetric services may be distributed to the same neighborhood, the bandwidth of the telephone cable should be allocated in a way which supports both symmetric and asymmetric services. Additionally, since the customers for these services may be distributed throughout a neighborhood, it would be highly desirable to provide symmetric and asymmetric services of different reaches. However, since attenuation and far end cross talk are more pronounced at higher frequencies, the spectral utilization of a channel which affords longer reach is preferably concentrated at lower frequencies. If all the end users supported by a cable were the same distance from the cable's point of origin and all required exactly the same services, allocation of the cable's frequency bands would be a fairly straightforward matter. When supporting services of various symmetries and loops of various lengths, however the proper allocation of frequency presents a daunting task.